Stars, galaxies, and other visible stuff in the universe only make up a tiny fraction of what's out there. The rest consists of the more mysterious dark matter and dark energy.

Scientists infer the presence of dark matter by the way it distorts the light of distant galaxies that pass through it on the way to observers. A circular galaxy, for example, may appear elliptical ... or even as curved as a fingernail clipping. The technique, called gravitational lensing, allowed scientists to infer the presence of dark matter in the giant galaxy cluster Abell 1689, as mapped in the image above.

But dark matter doesn't distort all galaxies equally. Unlike the obvious distortions in the Hubble image, the effect is often "so small that you can't really see it by the eye," challenge organizer Thomas Kitching from the University of Edinburgh told me. "So we need to do it statistically."

Astronomers want to measure this lensing effect in 52 million galaxies. An additional layer of complexity arises from the blurring of images due to other distortions from the atmosphere and the telescopes themselves.

"The challenge is to undo the blurring effect of the atmosphere and the telescopes, and get back to measuring the very slight distortion. And if algorithms and software can be developed to measure that, it then means we can directly use those algorithms to map out the dark matter," Kitching said.

The scientists ultimately hope to map out dark matter in the universe as a function of time. That would let them see how the structure of dark matter has changed as the expansion of the universe has accelerated due to an effect of another dark force -– dark energy. Astronomical observations suggest that ordinary matter accounts for just 4 percent of the universe's content, and that dark matter takes in another 25 percent or so.

"We can actually say something about dark energy, which accounts for the other 70 percent of the universe and is causing the accelerated expansion," Kitching told me.

The challenge is open to anyone, though organizers are particularly keen for citizen scientists with experience in image manipulation and software development to step up to the plate -- for instance, the kind of people behind Galaxy Zoo, another online science project.

Kitching also would love to hear from people who have an idea but are not sure how to express it mathematically or with software. "If we think it is a good idea, then we are happy to work with them and turn their idea into a method that we can test," Kitching added.

Participants who download the GREAT10 data analysis package for the "Galaxy Challenge" will have nine months to run the simulations and process imaging data. The winning teams will receive an iPod or iPad, as well as an all-expenses-paid trip to NASA's Jet Propulsion Laboratory in Pasadena, Calif., for one of the team members. JPL is where organizers will meet for a workshop on the GREAT10 project in September 2011. (Check out the project FAQ for details.)

In addition to the prizes, the winners will also get the feeling "that they helped us understand the dark matter and dark energy," Kitching added.